Gay marriage is increasingly accepted precisely because its results, to date, have been good for society. Polyamory on a large scale would have negative short-term results and that's a good reason to think it's just not going to happen.
The morality of having multiple partners, specifically where there is one man for multiple women, will always be in question. One recurring statement that is heard around these relational issues is: "It's their choice."
Without revealing too much of the plot of Sweet Land of Bigamy, I will say that it is a true blessing to live in a time, and a country, where a woman, even in fiction, could marry two men and when found out, not immediately led to the gallows.